Oxynitride fuel kernel for gas-cooled reactor fuel particles

ABSTRACT

A new nuclear fuel for use in gas-cooled reactors comprising a carbon coated kernal of UO2-xN3/4 x where 0 &lt; x &lt;/= 1.

United States Patent [1 1 Leitnaker et al.

[451 Apr. 15, 1975 OXYNITRIDE FUEL KERNEL FOR GAS-COOLED REACTOR FUELPARTICLES Inventors: James M. Leitnaker, Kingston;

Terrence B. Lindemer, Oak Ridge, both of Tenn.

The United States of America as represented by the Administrator of theEnergy Research and Development Administration, Washington, DC.

Filed: Aug. 8, 1973 Appl. No.: 386,657

Assignee:

US. Cl. 176/67; 176/82; 176/91 SP; 252/30l.l R; 264/O.5 Int. Cl G2lc3/06; C2lc 3/20 Field of Search... 176/67, 82, 89, 91 R, 91 SP, 176/90;252/30l.1 R; 264/O.5

[56] References Cited UNITED STATES PATENTS 3,230,177 l/l966 Blum et al252/30l.l R 3,36l,638 1/1968 Bokros et al .4 1761/91 R 3,746,616 7/1973Leitnaker et a1 252/30l.l R 3,798,123 3/1974 Lindemer 176/67 PrimaryExaminerBenjamin A. Borchelt Assistant Examiner-C. T. Jordan Attorney,Agent, or Firm-John A. Horan; David S. Zachry; John B. Hardaway [57]ABSTRACT A new nuclear fuel for use in gas-cooled reactors comprising acarbon coated kernal of UO N I where O x s l.

6 Claims, 2 Drawing Figures OXYNITRIDE FUEL KERNEL FOR GAS-COOLEDREACTOR FUEL PARTICLES BACKGROUND OF THE INVENTION This invention wasmade in the course of, or under, a contract with the United StatesAtomic Energy Commission. It relates generally to a new fuel for use ingascooled reactors.

As is disclosed in commonly assigned copending ap plication Ser. No.235,206 filed Mar.l6, I972. now U.S. Pat. No. 3,798,123, fuel used ingas-cooled nuclear reactors is subject to coating thinning and theamoeba effect due to oxygen released during the fissioning of a UO fuelkernel. A typical fuel particle is sectionally shown in FIG. I of thedrawings. Such a particle comprises an approximately spherical oxidekernel (l a cover of porous carbon (2), a sealer layer of densepyrolytic graphite (3), a layer of silicon carbide (4) and a final layerof dense pyrolytic graphite (5). The inner or buffer layer of porouscarbon (2) with a porosity about to 70% absorbs any expansion orswelling of the kernel (I and minimizes damage to the other layers dueto fission fragment recoil from the kernel. The adjacent dense graphitelayer (3) is applied to isolate the kernel and layer (2) from attack bydeleterious gases, such as chlorine, used in depositing the siliconcarbide layer (4). The silicon carbide layer (4) gives dimensionalstability to the overall fuel particle and provides containment formetallic fission fragments. The silicon carbide layer may be omitted inexperimental fuels but is included in all present particles forpractical nuclear application.

Failure of the above fuel particle design during operational conditionshas been attributed to oxygen release during fissioning so as to produceCO and CO by reaction with the carbon buffer layer. This reaction hasbeen observed to cause a thinning of the coatings and in some cases amigration of the kernel out of the coatings. The latter phenomenon isknown as the amoeba effect. In either instance, the failure of theparticle is attributable to excessive CO and CO pressures within theparticle as a result of oxygen released during fissioning of the fuel.

SUMMARY OF THE INVENTION I It is thus an object of this invention toprovide a new fuel particle for use in gas-cooled reactors.

It is a further object to provide a fuel particle which accommodatesoxygen released during fissioning so as to minimize CO and CO pressureswithin the fuel particle.

These, as well as other objects, are accomplished by utilizing a fuelkernel comprising UO N I where O x 1 within a conventional fuelparticle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a sectional view of a HTGRfuel particle. FIG. 2 is a ternary phase diagram of the uranium, oxygen,nitrogen system.

DETAILED DESCRIPTION According to this invention, it has been found thatUO ,N I where O x l maintains a low oxygen 2 of the drawings. This is asingle phase region which contains no UN. At elevated temperatures, UO N(area B) will form, thus causing the composition in area ,A to movetoward U0 However, this reaction has no adverse effect on the gas phaseequilibria which is of interest to this invention.

In general, the fuel kernel of this invention comprises either aphysical mixture of U0 and U N or the solid solution formed thereby.Under operational conditions U0 and U N will normally not existindividually but will exist as the solid solution UO N I which is formedby Either state, however, is effective to establish the desiredequilibrium and thus maintain the CO and CO pressures at a level belowthat which will cause fuel failure.

The apparent reason that the fuel of this invention reduces fueldegradation due to the presence of CO and CO within the fuel kernel isthat oxygen released during fission is accommodated into the UNO systemby gas-solid equilibrium reactions. Simply stated, this accommodation isexpressed by The brackets in the above equation are used forthermodynamic convenience to indicate the presence of the compounds inthe single-phase solid solution. In practice, the equilibrium of oxygenwith carbon monoxide and carbon is also maintained via the reaction(Ill) and the addition of these two reactions gives [UN] 2C0: [U0 2C %NThe latter equation represents equilibrium between all the major solidand gaseous phases present in the fuel particle. The presence of minoramounts of CO is established by the equilibrium 2C0 CO C in which P isgenerally P In either event the 0 CO and CO pressures are maintained ata level at which fuel failure will not result from coating thinning orthe amoeba effect. It should be noted that nitrogen is released as aconsequence of removing oxygen from the kernel atmosphere; however,nitrogen does not deleteriously affect the carbon coatings.

The selection of the value x in UO N I is based on estimates of thechemical behavior of the fuel-fission product system within a coatedparticle. By considering the release of oxygen from U0 during fission, avalue of x can be preselected so as to accommodate the released oxygen.The oxygen release is not precisely known, but for the present purposesthe following expressions encompass the available estimates:

C 92 33103 M 0 0.30 CO C UO liifl 1.07 M 0 0.15 CO C U0 1.10 M 0, 0.01CO a. (l-.01F) moles UO -N v11) b. (.OlF) M/U moles of fission productoxide (Vlll) c. /2[%x%w(10lF)]=moles OfNz gas=NN 1x d. negligible molesof CO or CO gas e. no solid-phase nitrides The conservation of massrequires that (moles of oxygen) initial (moles of oxygen) after burnup(moles of nitrogen) initial (moles of nitrogen) after burnup.

Use of these relations to solve for w gives in which M/U is the ratio offission product metal oxide formed per uranium fission, Equation VI, andO/M is the average ratio of oxygen-to-fission product metal in thefission-product oxides, Equation V1.

Equation X can thus be used to calculate the minimum value of xnecessary in the original oxynitride fuel. A requirement here is thatthe value of w after a given final burnup, F,, be no less than zero.Equation X is thus used for w 0 to give iniliril Typical minimum valuesof x are given in Table I.

TABLE I Typical Minimum Values of x (M/U) (O/M) xatF= (Eq. v1 25 50 75100(theoretical) The maximum initial value ofx should be about threetimes the minimum allowable value to compensate for any errors in theestimation of the chemical behavior of the system. From Table l, themaximum value of x can be calculated to be 0.9.

The fuel particle of this invention may be produced by either formingmicrospheres ofa mixture of U0 or U N with about 0.01 to 5.2 weightpercent'of U N or preferably by forming the solid solution frompreviously formed U0 microspheres.

The solid solution is best formed by reacting U0 microspheres withnitrogen to form UO N This is done by placing UO in a bed of carbon andheating in the presence of N and CO. Since x moles of oxygen are removedfrom the U0 to form UO N it follows that at least x moles of carbon mustbe present to remove the oxygen as CO. Table 11 lists fabricationconditions which can be used to produce the fuel of this invention withthe desired value of x.

TABLE 11 Range of x in Exact T P P UO -;,.u x "C Torr Torr Preparationof the UO N I is typically accomplished by a variation of the processdisclosed in U. S. Pat. No. 3,510,434. Microspheres of U0 prepared by aconventional method such as the sol-gel technique are used as feedmaterial. These microspheres may have an O/U ratio of exactly two orthey may be hyperstoichiometric (O/U 2 y) or hypostoichiometric (O/U 2y) in oxygen. The microspheres are mixed with carbon or graphiteparticles, the amount being at least [(Zi'y) (2x)] moles of graphite permole of U02 y- The carbon particles are conveniently a mesh size that issmaller than the U0 so that excess carbon may be removed after heattreatment by a screening process. Excess carbon may be added to thesystem and, in fact, this is desirable in order to maximize the UO-carbon contact during the heat treatment; such close proximity isnecessary so that oxygen removed from the U0 can react immediately withthe carbon to form CO.

The UO -carbon mixture is placed in a graphite, tungsten, or molybdenumvessel and is placed in a furnace. A gas mixture containing CO and N atthe desired partial pressures (Table l) is flowed through the furnace ata given temperature (Table l) and at a rate that insures thatapproximately ten volumes of CO and N are swept past the UO -carboncharge for each volume of CO removed from the U0 For example, if onemole of UO l I were produced, then 0.4 mole of CO is released, or 8.96liters of CO at STP. Thus, about 90 liters (STP) of C0 N should beflowed past the charge during the heat treatment. This is done to insurethat the CO/N ratio remains at the value necessary to give the desiredvalue of at. Other gases may also be added to the CO and N gases; argonis conveniently added to provide a total gas pressure of one atmosphere.Hydrogen additions in the amount of 1-8 volume percent are alsobeneficial because this procedure is generally known to enhance thegas-phase transport of nitrogen, carbon and/or oxygen between the U0 andcarbon.

The time necessary for the processing is about one hour at 1,700C andabout hours at l,200C for 1,000 micrometer U0 microspheres. The time isindependent of the amount of U0 charged.

After the desired reaction time, the charge should be cooled to ambienttemperature as rapidly as is practicable (e.g., furnace turned off) tominimize changes in x during cooling. Removing the crucible and chargefrom the hot zone of the furnace is even more desirable. The gas flowcan then be stopped. After the charge is at ambient temperature theexcess carbon particles can be removed by screening.

Many variations of the above process are possible. For example,resin-derived microspheres may be used with excess carbon in themicrospheres providing the necessary carbon content. The reaction mayalso be carried out in a fluidized bed.

Coating layers may be applied by conventional techniques. The initiallow density, highly porous carbon coating may be applied, for example,using the method disclosed in U. S. Pat. No. 3,472,677. A high densitycoating may then be applied using the method disclosed in U. S. Pat. No.3,471,314. A SiC layer, if desired, may be applied in a fluidized bedwith the SiC being derived by the thermal decomposition of, for example,methyl trichlorosilane. The outer carbon layer may be produced by againusing the process of U. S. Pat. No. 3,471,314.

EXAMPLE The pressure of carbon monoxide (P in a pure U0 HTGR particlecan be obtained from an adaptation of the ideal gas law in which where aratio, void volume in kernel and buffer layer/- superficial volume ofkernel 0 molar volume of U0 R 82.06 cm"-atm/mole K T temperature, "K

F percent FIMA N moles of carbon monoxide in Equation (VI).

This equation was used to calculate the values of P in Table III as afunction of burnup at 1,800K with a typical value ofa 2.5. As acomparison, the values of P and P in the fuel particle of this inventionare included. The pressure of N during burnup is calculated from themoles of nitrogen released, which is obtained from relations (IX) and(X). This is (Xlll) The P during burnup is calculated from the relationfor the equilibrium constant of reaction (IV); this is approximately C0initial Gas Pressures During Burnup of U0 At Approximately l800K and a2.5

Fully-enriched UO ,N;, 235 x 0.5 PX! PK! PFO PIU PS2 71 FlMA Atm. Atm.Atm. Atm.

As is seen from the above example, the fuel particle of this inventionprovides a superior advance over the prior art U0 fuel particles in thatthe P is greatly reduced, thus minimizing fuel failure due to coatingthinning and the amoeba effect.

While this invention has been explained with reference to U0 and U N itis equally applicable to mixed oxide-nitride systems in the U-Pu, U-Th,and Th systems.

What is claimed is:

1. A high temperature gas-cooled reactor fuel particle comprising aspheroidal kernel of a metal oxide and a metal nitride with up to 33%;mole percent nitride, said oxide and nitride being selected from thegroup consisting of a mixture of U0 and U N a mixture of (U,Pu)O and U Na mixture of ThO and Th N and a carbon cover adjacent said kernel.

2. The particle of claim 1 wherein said oxide and nitride are U0 and U N3. The particle according to claim 2 wherein said U0 and U N are presentas a solid solution UO N wherein 0 x l.

4. The particle according to claim 2 wherein said U0 and U N are withinthe area A of FIG. 2.

5. The particle according to claim 2 wherein said U N is present in anamount of from 0.06 to 22- /2 mole percent.

6. The particle according to claim 2 further comprising a dense graphitelayer adjacent said carbon cover, a SiC layer adjacent said graphitelayer and a second dense graphite layer adjacent said SiC layer.

1. A HIGH TEMPERATURE GAS-COOLED REACTOR FUEL PARTICLE COMPRISING ASPHEROIDAL KERNEL OF A METAL OXIDE AND A METAL NITRIDE WITH UP TO 33-1/3MOLE PERCENT NITRIDE, SAID OXIDE AND NITRIDE BEING SELECTED FROM THEGROUP CONSISTING OF A MIXTURE OF UO2
 2. The particle of claim 1 whereinsaid oxide and nitride are UO2 and U2N3.
 3. The particle according toclaim 2 wherein said UO2 and U2N3 are present as a solid solutionUO2-xN3/4 x wherein 0 < x <
 1. 4. The particle according to claim 2wherein said UO2 and U2N3 are within the area A of FIG.
 2. 5. Theparticle according to claim 2 wherein said U2N3 is present in an amountof from 0.06 to 22- 1/2 mole percent.
 6. The particle according to claim2 further comprising a dense graphite layer adjacent said carbon cover,a SiC layer adjacent said graphite layer and a second dense graphitelayer adjacent said SiC layer.